So apparently you can buy devices that transfer internet through your power sockets. That is, you can theoretically plug your modem into an electrical outlet downstairs, and then connect a computer to the internet through any electral outlet upstairs without a wireless router.
Seriously, you can plug just about anything into your walls.
According to Wikipedia, it’s a well-known design called Powerline Communication. Neato!
I found a brief explanation on Quora:
to make it short and simple, two signals are combined, a low frequency 50 or 60 Hz, high amplitude (120V) power signal and a high frequency (100 MHz) low amplitude (around 1 volt or less) communications signal.
They can be combined in the powerline adapter. At the other end, the signals are separated with a high pass filter set. the huge separation in frequency makes it easy to split them efficiently. The high frequency signal is then decoded for data and the low frequency part can be used elsewhere for power without removing the high frequency as most appliances will not even see it.